1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus capable of insuring user's safety when the apparatus is not used.
2. Description of the Background Art
It is a common practice with an image forming apparatus to use a fixing device of the type having a heat roller heated by a heat source and a press roller pressed against the heat roller. In this type of heating device, while the heat roller being heated is in rotation, a sheet or recording medium carrying a toner image thereon is passed through a nip between the heat roller and the press roller and has the toner image fixed thereon by heat and pressure. A heater for heating the heat roller has customarily been implemented by a halogen heater using a halogen lamp.
A current trend in the imaging art is toward a power saving type of image forming apparatus constructed to shut off power supply to a heater when the apparatus is not used from the environment standpoint. In this type of image forming apparatus, it is necessary for the heater to immediately raise the surface temperature of a heat roller to preselected temperature at the time of image formation. In practice, however, power is applied from a commercial power supply to one or more heaters, so that power available for the individual heater is limited. Moreover, a certain period of time is necessary for a halogen heater to heat the heat roller to the preselected temperature.
In light of the above, it has been customary with a fixing device of the type using a halogen heater to reduce the wall thickness of a heat roller to 1 mm or below. Such a small wall thickness is successful to reduce the thermal capacity of the heat roller and therefore to sharply heat the heat roller to preselected temperature. The small wall thickness, however, brings about the following problems. A heat roller with a thin wall lacks mechanical strength and is therefore apt to collapse, warp or otherwise deform. Further, as the wall thickness of the heat roller decreases, machining becomes technically more difficult to practice and requires higher accuracy, resulting in an increase in cost.
To solve the problems stated above, there has been proposed a fixing device configured to apply power greater than power available with a commercial power supply to a heater, thereby accelerating the warm-up of a heat roller. This is contrastive to the above-stated scheme that reduces the thermal capacity of a heat roller in order to reduce power consumption losses. Japanese Patent Laid-Open Publication No. 3-36579, for example, discloses a heating device for a fixing device that includes a heater configured to generate heat on receiving power via heater drive means. The heater drive means includes a rechargeable battery and a charger connected to a commercial power supply for charging the battery. The heater is made up of a main heater receiving power from the commercial power supply and an auxiliary heater receiving power from the battery. The battery is selectively connectable to the charger to form a charge circuit or connectable to the auxiliary heater to form a discharge circuit. The discharge circuit serves to reduce the warm-up time of the heater.
Japanese Utility Model Laid-Open Publication No. 63-150967 teaches a fixing device including heating means implemented as a first heater powered by an AC power supply and a second heater powered by a battery rechargeable by charging means.
Japanese Patent Laid-Open Publication No. 3-5779 discloses an image forming apparatus including a thermal fixing device in which a heat roller accommodates a main heater and a subheater therein. More specifically, the apparatus taught in this document includes a main power supply for heating the main heater, first switching means for selectively turning on or turning off the main power supply, a battery for heating the subheater, charging means for charging the battery, second switching means for selectively connecting the battery to the subheater or the charging means, temperature sensing means responsive to the temperature of the heat roller, and control means for controlling the first and second switching means in accordance with the output of the temperature sensing means. When the temperature of the heat roller being heated by the main power supply drops to reference temperature related to a fixing ability, the subheater is heated via the battery. When the temperature of the heat roller rises above the reference temperature, the heating of the subheater is interrupted.
Japanese Patent Laid-Open Publication No. 2000-315567 proposes a heating device including a heating section and a main and an auxiliary power supply unit. The heating section includes a heat generating body to be applied with power from the main and auxiliary power supply units. The auxiliary power supply unit is implemented by a capacitor.
Japanese Patent Laid-Open Publication No. 2000-98799 discloses a heating device for a fixing device that includes a heater for generating heat when applied with power and heater drive means for feeding power to the heater. The heater drive means includes a rechargeable battery and a charger for charging the battery by being applied with power from a commercial power supply. The heater is made up of a main heater powered by the commercial power supply and an auxiliary heater powered by the battery. The battery is charged when the main heater is turned off.
The precondition with any one of the conventional heating devices, fixing devices and image forming apparatuses discussed above is that the capacitor, rechargeable battery or similar auxiliary power supply can store electric energy great enough to implement rapid warm-up. Despite that such a precondition involves risks as to user's safety, no safety measures have been taken with the conventional devices as to troubles ascribable to the auxiliary power supply. Particularly, when the image forming apparatus enters a non-use mode with great energy remaining in the auxiliary power supply, dangers unexpected with the conventional image forming apparatus are apt to occur. The non-use mode mentioned above refers to a condition wherein the apparatus is transported for disposal or maintained by way of example.